Special interests have pumped more than a half-million dollars into San Francisco's district supervisor races - and the amount is expected to balloon as election day nears.

Under city law, individual donors, whether a person, union, organization or business, can give no more than $500 to a candidate's campaign. But those same donors can give as much as they want to independent expenditure committees. The money funds mail, television, phone-bank and ground operations that promote - or vilify - candidates.

The spending comes in varying amounts, from a couple of hundred dollars to tens of thousands. And it has one purpose: to influence the outcome of the election.

"Money is influential in campaigns. No doubt," said Charles Marsteller, a longtime campaign finance watchdog. That makes it all the more important, he said, to have a strong disclosure law and a muscular enforcement system so voters can know who's behind the spending.

By law, the independent expenditure committees are supposed to be just that, independent of the campaign operations controlled by the candidates. But that doesn't mean they don't often use the same photos, color schemes and messaging.

With four hotly contested races for the Board of Supervisors in the Nov. 2 election - a fifth race for District Four supervisor has incumbent Carmen Chu running unopposed - downtown business groups, real estate interests, tenant activists and labor organizations have been writing big checks.

The outcome of the races will determine the balance of power between the left-leaning lawmakers, who now hold the majority, and the moderates on the 11-member Board of Supervisors. Policies ranging from taxation and preservation of city jobs and services to development and law enforcement could be affected.

"Our priority for the city is jobs and to build the local economy, and we don't believe the current Board of Supervisors has focused on that, and we want candidates elected who will make that a top priority," he said.

His organization, which represents the commercial real estate industry, has aimed its resources at Steve Moss in District 10, Theresa Sparks in District Six, Scott Wiener in District Eight and Janet Reilly in District Two.

Service Employees International Union Local 1021, the largest city employees union in San Francisco, spent on behalf of Rafael Mandelman, who is running to the left of Wiener in the contest to represent the Castro, Noe Valley and Glen Park. Labor also has aided District Six candidate Debra Walker in her quest to represent the Tenderloin, North Mission and South of Market areas.

Gabriel Haaland, political director for SEIU Local 1021, said the union wants to help the candidates "with a pro-worker perspective" and a strong interest is preserving city services.

The San Francisco Tenants Union spent money on behalf of Mandelman and in opposition to Sparks, Wiener and Moss. New Day for SF, a committee with financial backing from former Mayor Willie Brown, put out a mail piece that promoted District Six candidate Jane Kim. The San Francisco Ethics Commission is investigating a complaint that the committee improperly colluded with Kim's campaign. Kim and her campaign manager say there was no wrongdoing.

While Reilly got help from the Realtors association, the San Francisco Democratic Party and the building owners association, money also has been spent against her. A new committee, "Common Sense Voters SF 2010," funded in large part by real estate investor Thomas Coates, spent more than $50,000 hoping to defeat her.

Coates, who has worked to repeal rent control in the state, opened his checkbook to support candidates he backs: $100,000 on Farrell, Reilly's chief rival; $45,000 each on Moss and Sparks; and $10,000 on Wiener.

In all, the independent expenditures now hover around $600,000, according to filings with the Ethics Commission. The bulk has gone to seven candidates out of the 46 running for the five seats on the ballot.

Ethics Commission chief John St. Croix said he expects to log a rush of independent expenditures in the final days of the campaign season. It is during that period when voter interest is heightened and time is running short for campaigns to respond to hit pieces.